The present invention relates to aircraft instrumentation, and more particularly to airspeed measurement systems for rotary wing aircraft.
Aircraft air-data information is required for aerodynamic control. Typically, aircraft speed is measured by pitot-static systems that provide indicated airspeed, that is, the ram or dynamic pressure compared to the static pressure for the particular altitude. The indicated airspeed provides information to the pilot for aerodynamic speed control.
Indicated airspeeds are dependent on air density and may be significantly different from true airspeed. Conventional systems generally measure indicated airspeed and use an air data computer or other calculating means to determine true speed. Several problems arise with existing devices as measurement accuracy is dependent on the alignment of the pitot head with the incoming airflow. At large yaw angles or at high angles of attack, significant errors may also be introduced. Additionally, the operating range of the pitot-static system is limited such that at low air speeds, the dynamic-static pressure differences may be too small for accurate readings.
Numerous devices have been developed to measure the low speed operation of rotary wing aircraft. Typically, these devices calculate indicated airspeed using complex computations of main rotor longitudinal and lateral cyclic pitch positions, along with collective pitch and tail rotor pitch. These devices typically require additional input data, such as inertial acceleration, inertial velocity, and wind velocity. The complexity, cost and weight of these types of systems have prevented the widespread acceptance of these systems.
Accordingly, it is desirable to provide an uncomplicated air-data system for a rotary wing aircraft that avoids the utilization of static pressure sensors while assuring accurate data over a broad range of airspeeds and directions.
NOMENCLATUREPT =Total PressurePT0 =Steady State PressurePS =Static Pressureγ =Ratio of Specific Heatρ =Densityψ =Phase angleVT =Air SpeedVR =Rotor VelocityPT1 =First Harmonic Pressure PulseTs =Ambient TemperatureR =Gas ConstantG =Gravitational AccelerationNr =Rotor SpeedMMA =molecular mass of airT =absolute temperaturePS =static pressureNR =rotor rotational velocity1P =blade crossing reference